It is found the when you download the PHP PAYPAL PHP API SDK from https://github.com/paypal/PayPal-PHP-SDK

The SAMPLE folder doesn’t make it to ZIP container.  This issue was reported here

https://github.com/paypal/PayPal-PHP-SDK/issues/1105

We have since pushed out newer version of the REST PHP SDK. This sample code is no longer applicable. Can you please try out the new 2.0 version instead?

And the above is found on the same page.

So SAMPLE is no longer valid and you can use new BETA version for SAMPLE here https://github.com/paypal/PayPal-PHP-SDK/tree/dev-2.0-beta